Use of heart rate monitors has become quite popular during indoor and outdoor recreational exercise. One popular heart rate monitor has a chest strap component and a wrist worn component. The chest strap component contains electrodes which detect an electrocardiogram (EKG) signal for the subject wearing the strap and circuitry which transmits detected signals. The wrist band component receives signals, derives heart rate data from the signal and displays heart rate information derived from the received heart rate data.
In particular, the chest strap portion receives an EKG signal from a subject, amplifies the signal and converts the signal into a pulse. The pulse is modulated by a 5 KHz clock and transmitted wirelessly. Thus, the transmitted signal has a 5 KHz pulse and is typically between five to twenty milliseconds in pulse length. The heart rate receiver module receives modulated signals, demodulates the signals, limits the signals and amplifies the received signal. The amplified signal is then processed to convert the signal back into a digital pulse.
When processing signals in the 5 KHz range, the receiver is prone to receiving considerable amounts of noise and crosstalk. Examples of noise that may be picked up include, for example, fluorescent lighting, vibrations of the subject and surrounding objects, audible noise from audio equipment and other sources, and other energy sources. A receiver may also experience cross-talk between multiple user chest straps. Cross-talk can occur when a receiver picks up signals transmitted by more than one user's chest strap and is undesirable because the receiver picking up two signals has difficulty determining a heart rate for a particular user. In particular, it can be difficult for a monitor to differentiate between a primary subject's signal and unwanted secondary subject signals. Though some chest strap transmitters are encoded, receivers at health clubs and at other locations usually don't recognize encoding. For example, if an exercise machine locked into an encoded chest strap signal, this could prevent subsequent users from utilizing the heart rate monitoring functionality for that exercise machine if they did not have a similarly encoded strap.
Thus, in a club environment for one-way data communication between a transmitter and a receiver, the 5 KHz receivers are generally not made to recognize encoded signals. As a result, the crosstalk is a common problem due to the close proximity between exercise machines and one machine can usually receive signals from multiple near-by heart rate transmitters.